In this revised application, Dr. Mosser proposes to study the mechanisms by which ligation of selected receptors on macrophages and dendritic cells influences the subsequent development of Type-1 and Type-2 responses from T cells. The hypothesis to be tested is that the ligation of some receptors on antigen presenting cells favors the production of cytokines that promote Type-1 responses, whereas ligation of other receptors may favor the production of cytokines that elicit Type-2 responses. Data in support of this hypothesis comes from a well-developed model system in which ligation of the FcR1 receptors on murine bone marrow-derived macrophages suppresses the production of IL-12 when these cells are later stimulated with LPS. This suppression is specific, because ligation of complement receptors does not lead to down-regulation of LPS-induced IL-12 production. Furthermore, this down-regulation of IL-12 expression is regulated at the transcriptional level. In contrast, ligation of the FcgR1 receptors leads to the enhancement of LPS-induced IL-10 production. These investigators have gone on to show that suppression of IL-12 production can occur by two distinct mechanisms, one which requires extracellular calcium fluxes and the other which is mediated by IL-10. Together, these two mechanisms may lead to diminished Type-1 responses, augmented Type-2 responses, and a suppression of macrophage pro-inflammatory responses in vivo. Both published and preliminary data are provided to demonstrate the capacity of FcgR1 ligation to suppress pro-inflammatory responses in several in vitro and in vivo models. The goal of this application is to determine the extent to which macrophage-and dendritic cell-derived cytokines can influence T-cell responses. There are four specific aims. The first aim will examine the cytokine profile of macrophages and dendritic cells following specific receptor ligation and stimulation via engagement of CD14 or CD40. The second aim will define on the intracellular events that suppress the production of IL-12 following receptor ligation. The third aim will examine the extent to which macrophages and dendritic cells can influence T cell responses to antigens in vitro. Antigens will be targeted to specific receptors on these APCs, and lymphokine production by defined T cell clones will be measured. The fourth aim will examine the extent to which macrophages and dendritic cells can influence T cell responses to antigens in vivo.